Michael Conforto swung and missed, crumpled to the ground and all but started his offseason.

The Mets outfielder is finished for 2017, after sustaining a dislocated left shoulder and tear in the joint’s posterior capsule — which may necessitate surgery — upon swinging and missing a pitch from Diamondbacks starter Robbie Ray in the fifth inning Thursday at Citi Field.

The Mets announced Conforto’s dislocated shoulder during a 3-2 loss at Citi Field, but another bombshell was dropped hours later, when the team said test results revealed a tear in the shoulder’s posterior capsule. In a release, the team said all treatments, including surgery, were under consideration.

After writhing in pain, Conforto was helped from the field by trainer Ray Ramirez and replaced by Brandon Nimmo.

And the mood in the Mets dugout reflected the scene.

“Quiet. A lot of sadness. Frustration,” Dominic Smith said. “It just kind of makes you feel depressed. It just [stinks] to see another player go down.”

Already this week the Mets have lost Steven Matz to season-ending surgery to relocate the ulnar nerve in his left elbow, and the team announced Thursday that Zack Wheeler — who hadn’t pitched in a month because of a stress reaction in his arm — was shut down for the remainder of the season.

Conforto, the Mets’ lone All-Star this year, was batting .279 with 27 homers and 68 RBIs in his breakout season.

Terry Collins and Michael ConfortoPaul J. Bereswill

“It turns your stomach,” Collins said. “A young player who was having a tremendous year and really making a name for himself and to go down like that with that kind of an injury is tough to watch.

“Ces is starting to heat up,” Collins said, referring to Yoenis Cespedes. “And with Michael in the lineup, you have got two real legitimate guys in the middle of your lineup. To have the year he’s had and all the work he’s put in, it’s just tough to see a guy like that get hurt in any situation.”

Rafael Montero (2-9) kept the Mets in the game by allowing three earned runs on seven hits and two walks over 5 ¹/₃ innings. It was a third straight respectable start for the right-hander, who has pitched to a 3.12 ERA over the last three weeks.

Rafael Montero reacts after allowing a run in the 6th inning.Charles Wenzelberg

Cespedes’ solo homer in the fourth inning gave the Mets a 1-0 lead before the Diamondbacks scored twice in their next at-bat. In the bottom of the inning, Conforto was ahead 2-0 in the count with runners on first and second when he swung and missed, damaging the shoulder.

“It wasn’t a crazy play,” Smith said. “It just shows how tough this game can be on our bodies.”

Conforto had a stint on the disabled list earlier this season, after getting drilled in the left hand with a pitch on June 25 in San Francisco. But Conforto returned the weekend before the All-Star break, in time to represent the Mets in the Midsummer Classic.

Once stockpiled with outfielders, the Mets — following trades that sent Jay Bruce and Curtis Granderson to the Indians and Dodgers, respectively — are reduced to Cespedes, Nimmo and Juan Lagares for the three spots. It’s possible the team will call up Travis Taijeron from Triple-A Las Vegas to provide depth. Matt Reynolds also has outfield experience and Jose Reyes could be added to the mix once he returns from the disabled list, most likely on Saturday.